This invention relates to a motor vehicle hydraulic fluid power circuit having a hydraulic pump for supplying fluid pressure to assist the operator in steering and braking the vehicle.
Motor vehicles are frequently provided with a hydraulic pump which is driven by the engine of the vehicle to provide a source of fluid power. In passenger cars, this source of fluid power may be used in a fluid power circuit to assist the operator in steering the vehicle and to assist the operator in braking the vehicle.
In such motor vehicle fluid power circuits, it is also desirable to provide one ratio of brake pedal movement to master cylinder movement under normal operating conditions and to provide a different ratio in the event of fluid pressure failure in the system. This is desirable because one such ratio will provide the brake pedal movement and brake pedal feel characteristics to which drivers are now accustomed with vacuum actuated braking systems, while a different ratio is required to provide high braking pressures with low pedal effort in the event of pump failure.
In order to provide one ratio of brake pedal movement to master cylinder movement during normal operating conditions and a second ratio in the event of pump failure, both mechanical and hydraulic ratio change devices have been proposed. The mechanical ratio change brake boosters include those which use lever type ratio changers such as shown in U.S. Pat. Nos. 3,733,966 and 3,733,968. Mechanical ratio change brake boosters also include spring type ratio changers such as shown in U.S. Pat. Nos. 3,714,869 and 3,751,919.